This invention relates to an apparatus utilizing guided wave radar for measuring the condition or characteristics of a material, and more particularly to improvements in level measurement.
Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few. Recent advantages in pulsed radar, also known as ultra-wide band (UWB) radar, in conjunction with advances in equivalent time sampling (ETS), permit development of low power and low cost time domain reflectometry (TDR) devices.
In a TDR instrument, a very fast stream of pulse with a rise time of 500 picoseconds, or less, is propagated down a transmission line that serves as a probe in a vessel. The pulses are reflected by a discontinuity caused by a change in impedance, such as at a transition between two media. For level measurement, that transition is typically where the air and the material to be measured meet. Alternatively, the transition could be two different liquids. The amplitude of the reflected signal depends on the difference between the dielectrics of the two media. The dielectric of air is one, while the dielectric of water is about eighty. The larger the difference in dielectric, the larger the reflected signal.
Guided wave radar is one technique available to measure the level of liquids or solids in an industrial environment using TDR principles. Guided wave radar works by generating a stream of pulses of electromagnetic energy and propagating the pulses down a transmission line formed into a level sensing probe. The probe is generally placed vertically in a tank or other container and the electromagnetic pulse is launched downward from the top of the probe. The probe is open to both the air and the material to be sensed in such a way that the electromagnetic fields of the propagating pulse penetrate the air until they reach the level of the material. At that point, the electromagnetic fields see the higher dielectric of the material. This higher dielectric causes a reduction in the impedance of the transmission line, resulting in a pulse echo being reflected back to the top of the probe. The pulse travels through the air dielectric portion of the probe at a known velocity. This allows the material level on the probe to be determined by measuring the round trip travel time of the pulse from the top of the probe to the level and back to the top of the probe. Conductive materials generate echoes similar to the echoes from high dielectric materials. Therefore, the same measurement technique also works with conductive materials.
Guided wave radar level measuring instruments may use time domain reflectometry for determining level. These instruments use both electrical and electronic circuits to determine level. Some such instruments use complex circuits for implementing the TDR techniques. The complexity of the circuits may require additional components increasing the costs of the resulting devices.
The present invention is directed to overcoming one or more of the problems discussed above, in a novel and simple manner.
In accordance with the invention there is provided an improved, relatively low cost guided wave radar measurement instrument.
There is disclosed in accordance with one aspect of the invention a process instrument comprising a housing and an active sensing element for sensing a characteristic of a process. The control circuit is disposed in the housing and is electrically connected to the active sensing element for measuring the sensed characteristic. A safety barrier comprises a blocking capacitor barrier electrically connected between the control circuit and the active sensing element.
The active sensing element may comprise a guided wave radar transmission line or a capacitance probe.
It is a feature of the invention that the blocking capacitor barrier comprises a plurality of series connected high voltage capacitors.
There is disclosed in accordance with another aspect of the invention a guided wave radar measurement instrument comprising a probe defining a guided wave radar transmission line. A pulse circuit is connected to the probe for generating a very fast stream of pulses on the transmission line and receiving reflected pulses returned on the transmission line. The reflected pulses represent a characteristic of a material being measured. An equivalent time sampling circuit is connected to the pulse circuit operable to sample reflected pulses to build a time multiplied picture of the reflected pulses and comprises a ramp generator circuit generating a saw tooth ramp signal used to selectively delay sampling reflected pulses to build the time multiplied picture. The saw tooth ramp signal has a controlled ramp start for each cycle and retrace at an end of the cycle. A processing circuit is connected to the equivalent time sampling circuit for selectively controlling ramp start for each cycle and measuring round trip travel time for a pulse from the pulse circuit.
It is a feature of the invention that the ramp generator circuit comprises a latching ramp comparator. The latching ramp comparator latches at the end of the cycle and is reset by the processing circuit to start the ramp for each cycle. The latching ramp comparator has an output coupled to a non-inverted input. The processing circuit resets the latching comparator by applying a low voltage to the non-inverted input.
It is another feature of the invention that the ramp generator circuit comprises a resistor network operatively controlled by the processing circuit for controlling slope of the saw tooth ramp signal.
There is disclosed in accordance with a further aspect of the invention a domain reflectometry measurement instrument comprising a probe and a pulse circuit connected to the probe for generating a very fast stream of pulses on the probe and receiving reflective pulses returned on their probe. The reflected pulses represent a characteristic of a material being measured. An equivalent time sampling circuit is connected to the pulse circuit operable to sample reflected pulses to build a time multiplied picture of the reflected pulses, comprising a ramp generator circuit generating a saw tooth ramp signal used to selectively delay sampling reflected pulses to build the time multiplied picture. The ramp generator circuit comprises a ramp comparator that latches at a start voltage during each cycle until receiving a start ramp signal and retraces to the start voltage at an end of the cycle. A processing circuit is connected to the equivalent time sampling circuit for selectively generating the start ramp signal for each cycle and measuring round trip travel time for a pulse from the pulse circuit.
Further features and advantages of the invention will be readily apparent from the specification and from the drawings.